It is well known that MRI enables the direct electronic visualisation of internal organs in living beings and is therefore powerful help and guide in prognosis, medical treatment and surgery. This technique can often advantageously supplement or replace X-ray tomography as well as the use of radioactive tracer compounds which may have obvious undesirable side effects.
The whereabouts of MRI techniques applied to the imaging of body organs are summarised in EP-A-0 502 814 and need not be developed in detail here; suffice to say that the useful parameters pertaining thereto, i.e. the relaxation time factors T.sub.1 and T.sub.2 of the water protons in the direct environment of the organs under investigation are usually not sufficiently differentiated to provide sharp images when the measurements are carried out in the absence of contrast agents. The differences of the relaxation time constants between protons in various parts of the organs can however be enhanced in the presence, in the environment of the hydrated molecules under excitation, of a variety of magnetic species, e.g. paramagnetic (which mainly affect T.sub.1) and ferromagnetic or superparamagnetic (which mainly affect the T.sub.2 response). The paramagnetic substances include some metals in the ionic or organo-metallic state (e.g. Fe.sup.+3, Mn.sup.+2, Gd.sup.+3 and the like, particularly in the form of chelates to decrease the intrinsic toxicity of the free metal ions). Ferromagnetic and superparamagnetic contrast substances preferably include magnetic particles of micronic or submicronic size, i.e. from a few microns down to a few nanometers, for instance particles of magnetite (Fe.sub.3 O.sub.4), .gamma.-Fe.sub.2 O.sub.3, ferrites and other magnetic mineral compounds of transition elements.
Until now, MRI contrast agents designed for imaging the digestive tract have mostly included solid magnetic materials generally in particulate form. This is so because to be effective, the contrast agents should more or less line the walls of the digestive tract, thus requiring bulk. Obviously paramagnetic species in water-soluble molecular form would not fit the foregoing requirements and, if used, they should be associated with bulk carriers.
For instance, EP-A-0 275 215 discloses MRI contrast enhancers for the investigation of the digestive tract comprising complexes of paramagnetic metal species like gadolinium, iron, manganese and the like associated with mineral particulate carriers such as alkaline-earth polyphosphates and apatite.
EP-A-0 083 760 discloses EDTA, DTPA and NTA chelates of paramagnetic metals chemically bonded to organic polymer carriers such as sepharose, dextran, dextrin, starch and the like.
Also in EP-A-0 299 920 there are disclosed complexes between paramagnetic metals such as Cr, Mn, Fe, Ni, Co, Gd, etc. and polysulfated oligosaccharides like sucrose or maltose, these complexes being used for MRI of the digestive tract.
U.S. Pat. No. 5,466,439 discloses diamidopolymers of conventional alkylene-aminopolycarboxylic chelatants such as EDTA, DTPA and the like, and their addition copolymers with methyl methacrylates. The structure of polyethylene diamide-DTPA obtained from ethylene diamine and DTPA dianhydride is given below for illustration: ##STR1##
The polymeric chelatants are used to immobilize paramagnetic metals, e.g. Cr, Mn, Fe and the lanthanides, e.g. Gd, and the complexes administered orally for imaging the digestive tract.
Although the achievements of the prior art have merit, there is still need for more performant internal paramagnetic MRI contrast agents, more particularly in regard to bioadhesivity and controlled transit time in the digestive tract. The present invention is a forward step in the right direction.